Master thesis defense by Helle Astrid Kjær

PHOSPHATE IN ICE CORES - finding a method for continuous detection of phosphate in ice cores

Abstract:

In the thesis defence the first method for detecting phosphate continuoulsy in ice cores will be presented.

Phosphorus is an important and sometimes limiting nutrient for primary production in the oceans. Because of deforestation and the use of phosphate as a fertilizer changes in the phosphorus cycle have occurred over the last centuries. On longer time scales, sea level changes are thought as well to have caused changes in the phosphorus cycle.

For the detection of phopshate in ice cores a continuous and highly sensitive absorption method was applied. It has previously been successfully applied for determining phosphate concentrations of sea water [Zhang and Chi, 2002]. The method was optimized for the low concentrations of phosphate expected in the ice cores. Parameters such as temperature and mixing length as well as reagent and buffer chemistry and others were investigated.
A line of melt water from the CFA melt head was mixed with a molybdate blue reagent and an ascorbic acid buffer. The absorption was measured at a wavelength of 710 nanometer in a 2 meter long liquid waveguide cell (LWCC) with an inner volume of 0.5 mL. The method has a detection limit of around 0.1 ppb and the resolution was approximate 2 cm, with a melt speed of 1.5 cm per minute, giving sub annual resolution in firn ice cores. A filter was inserted for removing dust, which otherwise gave optical interferences.

Results from two different ice cores will be presented; the NEEM S1 shallow ice core, which covers part of the last 120 yrs and the NEEM ice core in which parts covering the period 22.4 to 36.8 kyr B.P. was detected. Analysis of the phopshate signal as well as a discussion of further improvement of the method will be presented.